The present invention relates to semiconductors, and in particular, to metal semiconductor metal devices.
Optical detectors are used in fiber optic communications to translate optical signals into electrical signals. While optical detectors have traditionally been constructed as PIN diodes, another family of optical detectors incorporate metal semiconductor metal (xe2x80x9cMSMxe2x80x9d) technology. Detectors that incorporate MSM technology have certain advantages over PIN detectors. For example, MSM detectors exhibit less capacitance per unit area than PIN detectors. As a result, MSM detectors are more easily matched to the circuits in which they are used. In addition, MSM detectors are typically more easily fabricated.
A typical prior art MSM optical detector 10 is shown in FIGS. 1-3. FIGS. 1 and 3 show a cross-sectional, schematic-type view of the MSM optical detector 10 while FIG. 10 shows a top view. The prior art MSM optical detector 10 includes a GaAS absorption layer 12, a semi-insulating GaAs (xe2x80x9cSI GaAsxe2x80x9d) substrate layer 14, and two electrical contacts 16, 18. As shown in FIG. 3, the electrical contact 16 and the electrical contact 18 are metal Schottky contacts that comprise a set of interdigitated fingers 16a and 18a, respectively, and terminate in a connector pad 16b and 18b, respectively.
FIG. 3 illustrates the operation of the detector in the presence of a light signal and a bias signal. In operation, a bias signal, typically 5 volts, is applied across the contacts 16 and 18 via the connector pads, not shown in FIG. 3. As light strikes the GaAs absorption layer 12, positive carriers 17 and negative carriers 19 are generated and are swept to respective contact fingers 16a and 18a by the field established by the bias signal. The use of interdigitated fingers as shown in FIG. 2 increases the number of concentrated fields established on the semiconductor.
One factor that limits the usefulness of MSM semiconductors is their operational speed limitations. These speed limitations arise in part from penetrating light. Deep penetrating light causes either the semi-insulating (xe2x80x9cSIxe2x80x9d) substrate layer 14 or deep parts of the GaAs layer 12 to form carriers, referred to herein as deep carriers, that take a relatively long time to travel the contacts 16. As a result, electrical signal resulting from a light pulse may persist in a xe2x80x9cdecay tailxe2x80x9d that effectively limits the useful speed of the detector.
In particular, the response of a detector (such as the detector 10) may be measured in terms of rise and fall time in response to an applied light pulse. FIG. 4 shows a magnitude vs. time graph and includes an applied light pulse trace 20 and a typical device response trace 22. In general, the response trace 22 includes a primary response 24 and a decay tail 26. The positive carriers, or holes (h+), which have less mobility than the negative carriers, or electrons (exe2x88x92), cause the decay tail. The deep h+ carriers increase the decay tail 26 and thus slow down the effective response time of the device.
In order to increase the effective speed, U.S. Pat. No. 5,371,399 teaches a MSM detector that replaces the active GaAs layer 12 with a low temperature GaAs (LT GaAs) layer. The LT GaAs layer, due to its method of formulation, has relatively large xe2x80x9cclumpsxe2x80x9d or precipitates of Arsenic within its crystalline structure. The presence of precipitates provides recombination sites for deep carriers. The recombination of the deep carriers within the semiconductor has the effect of shortening carrier lifetime, which in turn increases the speed of the device.
While the LT GaAs layer improves response time, it undesirably decreases sensitivity of the device because it eliminates the carriers that would otherwise produce the response current in the contacts.
Accordingly, there is a need for an MSM detector that has improved speed over the traditional prior art MSM detector and improved sensitivity over the MSM detector shown in U.S. Pat. No. 5,371,399.
The present invention address the above-stated need, as well as others, by providing a MSM device that employs a wide-band gap layer below the absorption layer. The wide band gap layer does not typically produce any carriers, but rather transmits the light through to deeper layers within the device. Any carriers generated below the wide band gap Al GaAs layer (form the transmitted light) are blocked from returning upward to the contacts by the wide band gap layer.
One embodiment of the present invention is an MSM semiconductor circuit formed on a semi-insulating substrate that includes a set of contacts, first and second absorption layers, and a wide band gap buffer layer. The first absorption layer is formed on the semi-insulating substrate. The second absorption layer operably coupled to the set of contacts. The wide band gap buffer layer disposed between the first absorption layer and the second absorption layer.
Another embodiment of the present invention is a method of forming such a circuit. The method includes a first step of growing a first absorption layer on a semi-insulating substrate. The second step comprises growing a wide band gap buffer layer on the first absorption layer. Thereafter, a second absorption layer is grown on the wide band gap buffer layer. In addition, contacts are operably coupled to the second absorption layer.
The use of the additional absorption layer (i.e. first absorption layer) between the wide band gap layer and the semi-insulating substrate layer facilitates the absorption of the light transmitted through the AlGaAs layer. This absorption layer may simply be a normal GaAs layer, or may be a low temperature or intermediate temperature type of GaAs.
Because the wide band gap buffer transmits lights to layers deep within the device and does not generately transmit carriers from such layers to the surface, the wide band gap buffer substantially inhibits deep carriers from adversely affecting the response time of the device. Accordingly, in contrast to the prior art, the structure of the present invention does not depend upon the use of LT GaAs in the active absorption layer to recombine carriers and thus does not exhibit corresponding loss in sensitivity.